1. Field of the Invention
The present invention relates to a linear actuator including an actuator housing, a spindle rotatable in both directions, a threaded nut driving a piston rod, and a motor capable of driving the spindle through a transmission, a disengagement unit being arranged in the transmission to interrupt the connection between the motor and the spindle in case of operational failure, such as overloading of the spindle.
2. The Prior Art
Linear actuators are used for numerous tasks where a linear movement or application of a force is desired. Examples include electrically operated doors and hospital beds, where the linear actuator is arranged such that the piston rod performs the desired movement from one position to another as a result of control signals and optionally controlled by end stops.
In practice, it may be desirable to get from one extreme position to the other at a great speed or from a given position to one of the end positions, or, altogether, to be able to set the moved element without being tied to the mechanism of the linear actuator. In case of doors, e.g., it may be needed that the release of security means by an excessive temperature allows rapid closing as a fire door, or that a normally closed door must be capable of being used as an emergency exit even if the power supply should fail. In a hospital bed, it may be desirable rapidly to obtain a completely horizontal posture even though the head part of the bed is raised.
For this purpose, EP-A-0 685 662 discloses a linear actuator of the type mentioned where the connection between the spindle and the electric motor may be interrupted in an emergency. If the piston rod of the actuator is loaded, it will almost instantaneously be pushed (or pulled) back to the starting position.
The actuator of EP-A-0 685 662 includes means for disengaging spindle from motor. These consist of a disengagement unit between the spindle and a transmission shaft based on axially oriented coupling parts, which are connected with the spindle and the transmission shaft, respectively, and which are surrounded by a helical coupling spring capable of being expanded radially for disengaging the spindle from the motor.
In normal operation, the disengagement means are driven from the transmission. This drive is transferred through the coupling parts and further on to the spindle. If, on the other hand, the disengagement means are affected by a drive pulse from the spindle side, the spindle is disengaged from the motor transmission so that the spindle can rotate freely, which causes the piston rod to be pushed back to the starting position.
In many situations, however, this solution involves the risk of causing damage to the structure and the load carried by the actuator because of the impact applied to the structure and the load when the piston rod hits the end stop in the starting position.
The object of the invention is to provide a linear actuator which ensures that the structure and the load carried by the actuator are not damaged. It is moreover intended in this connection that the size and the cost price of the actuator are not increased noticeably, just as a solution must not reduce the efficiency of the actuator or increase the consumption of energy.